1. Field of the Invention
The present invention relates in general to a telephone system and pertains, more particularly, to a telephone system that is adapted to allow outside calls and extension calls to be made through the same telephone line.
2. Background of the Invention
Related art telephone systems adapted to allow both outside calls and extension calls are of two different types, one using an exchange and the other using no exchange. The telephone system of the former type uses, for example, a private branch-exchange (PBX), a small private branch-exchange (small PBX), or a key system unit (KSU) of a key telephone system or the like.
An example of a multi-line telephone system (having two outside lines 1 and 2) using a related art small branch-exchange or key telephone system is shown in FIG. 3. As is shown, an exchange E is provided at the interface between the outside lines (1 and 2) and the extension lines (1 and 2), so that there is no direct connection of the outside lines and the extension lines. A large number of telephone units are commonly connected to the two extension lines 1 and 2. The exchange E functions to electrically separate the extension lines from the outside lines during extension calls.
An example of the telephone system of the type with no exchange is a telephone system which includes a channel used exclusively for extension calls in addition to the telephone lines. Typically, this exclusive extension communication channel is a power line, with inter-extension communication being performed by imposing a signal on the power line.
In all of the above-described related art, telephone systems which are either of the type which uses an exchange or of the type which does not use an exchange, only one communication could take place on a single telephone line at a time. Particularly, an outside call and an extension call could not occur simultaneously on one line. Further, since a sole communication channel is used both for outside and extension calls, it has been necessary, in the system using an exchange, to use the exchange at the interface between the extension line and the outside line so that the extension line is electrically isolated from the outside line during extension calls. Use of this exchange involves installation and wiring work therefore making the telephone system itself very expensive. In this system, furthermore, it has heretofore been impossible to simultaneously make an extension call and an outside call when the same telephone line is used for the making of these calls. Therefore, the number of simultaneous calls is limited by the number of lines.
On the other hand, the system which uses no exchange has had a problem in that a communication channel used exclusively for the extension had to be provided in addition to and separately from the ordinary telephone line. To avoid additional wiring in the typical installation, the existing indoor AC power line is frequently chosen as the means for providing this additional signal line for extension call purposes. A problem with using an existing indoor AC power line as the exclusive extension communication channel was that the quality of sound in communication was degraded due to various sources of noise such as microwave ovens, personal computers, televisions and the like, which are being used increasingly these days.
Accordingly, an object of the present invention is to provide an improved, simple and economical telephone system which enables outside calls and extension calls to take place simultaneously over the same telephone line without the necessity to employ an exchange or to provide additional communication lines.
Another object of the present invention is to provide an improved telephone system of the type disclosed herein which can also be implemented with a multiplicity of lines.